1. Field of the Invention
The present invention relates to iontophoretic devices for delivering a drug or medicant to a patient transdermally, i.e., through the patient's skin, and more specifically relates to an iontophoresis drug delivery system and circuit therefor which limits the current or voltage provided to the patient's skin to a safe level should a component of the circuit fail.
2. Description of the Prior Art
Iontophoresis can be defined as the electrically driven application of drugs or medicants, in their ionic form, to the surface tissues of a patient. The application of electric current causes migration of ions into the tissue wherein such migration is proportional to the quantity of current applied through the iontophoretic system.
One of the major drawbacks of iontophoresis is skin irritation or burns which can occur due to high current levels. It is known that the impedance of a patient's skin can range from over 100,000 ohms to nearly 1,000 ohms, depending on the duration that the iontophoretic current is applied, the magnitude of the current which is being delivered, the location of the system on the patient's body, and other factors. In a system where the desired current level, which is determined in part by the drug administered to the patient, is 1 milliamp, a voltage potential of 100 volts would result if the skin impedance is 100,000 ohms. Such a voltage would cause undesirable sensations to the user.
Numerous attempts have been made to control the amount of current or voltage provided to a patient during iontophoresis. For example, U.S. Pat. No. 4,292,968 to Ellis discloses an apparatus for delivering constant current during ion therapy (iontophoresis) which will abruptly switch to delivering constant voltage when the voltage across the electrodes of the drug delivery device reaches a predetermined level. The circuit disclosed in the Ellis patent includes a voltage limiter 14 which is provided in shunt with the electrodes of the device for limiting the output voltage across the electrodes. The Ellis patent describes the voltage limiter 14 as functioning as a variable resistive path shunting the electrodes. When the electrode voltage is less than a predetermined voltage, the limiter 14 is stated to present a high resistance and all the current generated by the circuit is provided to the electrodes. However, when the voltage across the electrodes reaches the predetermined voltage, the resistance of the limiter 14 is stated to drop, drawing current that would have been provided to the electrodes.
One of the major disadvantages of the circuit described in the Ellis patent is that it is not failsafe. Should certain of the components of the circuit fail, it is possible for the Ellis circuit to deliver excessive current to the patient, causing skin irritation or tissue damage. For example, if the voltage limiter 14 failed such that it no longer acted as a variable resistor or no longer shunted the electrodes of the device, no voltage regulation would occur when the predetermined voltage across the electrodes is reached. The voltage across the electrodes could reach dangerous levels, resulting in skin burns or tissue damage.
Another iontophoresis device is disclosed in U.S. Pat. No. 4,141,359 to Jacobsen, et al. The Jacobsen, et al. patent discloses an epidermal iontophoresis device which is stated to be capable of maintaining a constant current through the epidermal tissue. To prevent excessive voltage build-up and the accompanying dangers of shock and burns, a comparator circuit monitors current flow and voltage across the electrodes of the device and automatically triggers an SCR shut down circuit when impedance readings are outside of predetermined limits.
As with the circuit disclosed in the Ellis patent, the iontophoresis device described in the Jacobsen, et al. patent is not safe if certain components fail. For example, if the SCR fails and becomes effectively an open circuit, it will no longer be capable of de-energizing the current source used in the circuit, resulting in burns, shocks and other dangerous effects of excessive current and voltage.